The present invention relates generally to solid fuel ramjet engines, and more specifically to a solid rocket motor construction which allows the burn rate to be controlled by variations in the oxidizer content.
In solid fuel ramjet engines, the burn rate characteristics of the solid fuel is a major determiner of the fuel flow rate. Generally, the fuel flow rate equals the product of: the burn rate, the burn area, and the fuel density.
Since the fuel density is fixed by the selection of the fuel, and the fuel flow rate is fixed by the initial requirements of the engine, the burn rate and burn area are inversely related. If the burn rate is low the burn area must be large.
It is common practice in the art to select fuels with low burn rates which allows a given amount of solid fuel to last longer. Since volumetrically efficient fuel grains are limited to small initial burn areas, a low burn rate often yields excessive burn area requirements until after some fuel is burned away. A technique to increase burn rate when the initial burn area is constrained, would greatly enhance the performance capabilities of the solid fuel ramjet.
The task of providing a variation in the fuel burn rates of solid fueled motors is alleviated, to some degree, by the techniques disclosed in the following U.S. Patents, which are incorporated herein by reference:
U.S. Pat. No. 3,159,104 issued to Hodgson;
U.S. Pat. No. 3,283,510 issued to Mangum et al; and
U.S. Pat. No. 3,656,304 issued to McCullough.
The references cited above propose combining a solid oxidizer with a solid fuel in a certain ratio to control the burn rate. The systems in these references each use a plurality of fuel and oxidizer grains separated by "inhibitors" to allow a type of "throttle" control. The reference Hodgson additionally shows use of a laminated structure of alternating layers of fuel and oxidizer.
While the references cited above are instructive, they do not describe ramjet engines. Current air-breathing missiles are typically as described in the U.S. Pat. No. 3,357,187 issued to Whitlock entitled "Ducted Rocket Motor", the disclosure of which is incorporated by reference. These ramjet systems include a solid rocket motor with oxidized booster propellants which accelerate the missile to ramjet velocities, typically between mach 3 and mach 8. At these velocities the unoxidized ramjet fuel is ignited in the ramjet chamber to propel the missile using oxygen obtained from the atmosphere.
From the foregoing discussion it is apparent that there remains a need to control the burn rate characteristics in solid fuel ramjet engines. The present invention is intended to satisfy that need.